What Did Viruses Ever Do For Us? The Evolution of “Good” Viruses

Professor John Carr - President of the Cambridge Philosophical Society Professor John Carr - President of the Cambridge Philosophical Society Professor of Plant Virology - Department of Plant Sciences Viruses generally get a very bad press. Deservedly so. Viruses cause serious epidemics and pandemics in humans and threaten food security by causing crop and livestock diseases. However, of the more than 1030 virus particles that exist at any instant, few endanger us, plants, or animals. Most infect bacteria and archaea and without these viruses Earth’s biosphere would collapse. Viruses are the largest group of emerging plant pathogens. But paradoxically, many confer benefits on plants that may increase survival and reproductive success. These apparently mutualistic effects may be conditional, for example, by aiding host survival under conditions of drought or cold or, more surprisingly, by helping plants attract pollinators. Certain viruses, notably ‘partitiviruses’ and ‘endornaviruses’ may have travelled so far on an evolutionary path as to become obligate plant symbionts. These viruses are transmitted exclusively via seed and pollen and are not infectious in a conventional sense (i.e., not transmitted by invertebrates, wounding etc.), and induce no obvious disease. It has been suggested that these ‘persistent’ viruses are mutualists and might be useful in crop breeding. Recent experiments exploring these ideas will be discussed.